Recent developments have occurred in the field of very small switches having moving liquid metal-to-metal contacts and that are operated by an electrical impulse. That is, they are actually small latching relays that individually are SPST or SPDT, but which can be combined to form other switching topologies, such as DPDT. (Henceforth we shall, as is becoming customary, refer to such a switch as a Liquid Metal Micro Switch, or LIMMS.) With reference to FIGS. 1-4, we shall briefly sketch the general idea behind one class of these devices. Having done that, we shall advance to the topic that is most of interest to us, which is an improved technique for fabricating a significant plurality of such switches on a collection of substrates.
Refer now to FIG. 1A, which is a top sectional view of certain elements to be arranged within a cover block 1 of suitable material, such as glass. The cover block 1 has within it a closed-ended channel 7 in which there are two small movable distended droplets (12, 13) of a conductive liquid metal, such as mercury. The channel 7 is relatively small, and appears to the droplets of mercury to be a capillary, so that surface tension plays a large part in determining the behavior of the mercury. One of the droplets is long, and shorts across two adjacent electrical contacts extending into the channel, while the other droplet is short, touching only one electrical contact. There are also two cavities 5 and 6, within which are respective heaters 3 and 4, each of which is surrounded by a respective captive atmosphere (10, 11) of a suitable gas, such as N2. Cavity 5 is coupled to the channel 7 by a small passage 8, opening into the channel 7 at a location about one third or one fourth the length of the channel from its end. A similar passage 9 likewise connects cavity 6 to the opposite end of the channel. The idea is that a temperature rise from one of the heaters causes the gas surrounding that heater to expand, which splits and moves a portion of the long mercury droplet, forcing the detached portion to join the short droplet. This forms a complementary physical configuration (or mirror image), with the large droplet now at the other end of the channel. This, in turn, toggles which two of the three electrical contacts are shorted together. After the change the heater is allowed to cool, but surface tension keeps the mercury droplets in their new places until the other heater heats up and drives a portion of the new long droplet back the other way. Since all this is quite small, it can all happen rather quickly; say, on the order of a millisecond, or less. The small size also lends itself for use amongst controlled impedance transmission line structures that are part of circuit assemblies that operate well into the microwave region.
To continue, then, refer now to FIG. 1B, which is a sectional side view of FIG. 1A, taken through the middle of the heaters 3 and 4. New elements in this view are the bottom substrate 2, which may be of a suitable ceramic material, such as that commonly used in the manufacturing of hybrid circuits having thin film, thick film or silicon die components. A layer 14 of sealing adhesive bonds the cover block 1 to the substrate 2, which also makes the cavities 5 and 6, passages 8 and 9, and the channel 7, each moderately gas tight (and also mercury proof, as well!). Layer 14 may be of a material called CYTOP (a registered trademark of Asahi Glass Co., and available from Bellex International Corp., of Wilmington, Del.). Also newly visible are vias 15-18 which, besides being as tight, pass through the substrate 2 to afford electrical connections to the ends of the heaters 3 and 4. So, by applying a voltage between vias 15 and 16, heater 3 can be made to become very hot very quickly. That in turn, causes the region of gas 10 to expand through passage 8 and begin to force long mercury droplet 12 to separate, as is shown in FIG. 2. At this time, and also before heater 3 begins to heat, long mercury droplet 12 physically bridges and electrically connects contact vias 19 and 20, after the fashion shown in FIG. 1C. Contact via 21 is at this time in physical and electrical contact with the small mercury droplet 13, but because of the gap between droplets 12 and 13, is not electrically connected to via 20.
Refer now to FIG. 3A, and observe that the separation into two parts of what used to be long mercury droplet 12 has been accomplished by the heated gas 10, and that the right-hand portion (and major part of) the separated mercury has joined what used to be smaller droplet 13. Now droplet 13 is the larger droplet, and droplet 12 is the smaller. Referring to FIG. 3B, note that it is now contact vias 20 and 21 that are physically bridged by the mercury, and thus electrically connected to each other, while contact via 19 is now electrically isolated.
The LIMMS technique described above has a number of interesting characteristics, some of which we shall mention in passing. They make good latching relays, since surface tension holds the mercury droplets in place. They operate in all attitudes, and are reasonably resistant to shock. Their power consumption is modest, and they are small (less than a tenth of an inch on a side and perhaps only twenty or thirty thousandths of an inch high). They have decent isolation, are reasonably fast with minimal contact bounce. There are versions where a piezo-electrical element accomplishes the volume change, rather than a heated and expanding gas. There also exist certain refinements that are sometimes thought useful, such as bulges or constrictions in the channel or the passages. Those interested in such refinements are referred to the Patent literature, as there is ongoing work in those areas. See, for example, U.S. Pat. No. 6,323,447 B1.
To sum up our brief survey of the starting point in LIMMS technology that is presently of interest to us, refer now first to FIG. 4 and then to FIG. 5. In FIG. 4 there is shown an exploded view 25 of a slightly different arrangement of the parts, although the operation is just as described in connection with FIGS. 1-3. In particular, note that in this arrangement (25) the heaters (3, 4) and their cavities (5, 6) are each on opposite sides of the channel 7. Another new element to note in FIG. 4 is the presence of contact electrodes 22, 23 and 24. These are (preferably thin film) depositions of metal that are electrically connected to the vias (19, 20 and 21, respectively). They not only serve to ensure good ohmic contact with the droplets of liquid metal, but they are also regions for the liquid metal to wet against, which provides some hysteresis in the pressures required to move the droplets. This helps ensure that the contraction caused by the cooling (and contraction) of the heated (and expanded) operating medium does not suck the droplet back toward where it just came from. The droplets of liquid metal are not shown in the figure.
If contact electrodes 22-24 are to be produced by a thin film process, then they will most likely need to be fabricated after any thick film layers of dielectric material are deposited on the substrate (as will occur in connection with some of the remaining figures). This order of operations is necessitated if the thick film materials to be deposited need high firing temperatures to become cured; those temperatures can easily be higher than what can be withstood by a layer of thin film metal.
Also, if the layer of thin film metal is to depart from the surface of the substrate and climb the sides of a channel, then it might be helpful if the transition were not too abrupt. This may be arranged by staggering the positions (as in a staircase) of the edges of successive printed layers of the dielectric material as they are deposited to achieve an aggregate layer of a desired thickness.
FIG. 5 is a simplified exploded view 26 of a LIMMS device whose heater cavities, liquid metal channel and their interconnecting passages are formed in facing patterned layers of dielectric material (28, 30) between two substrates (27, 31), instead of being recesses in a cover block. The figure shows a portion of the two substrates 27 and 31, which may be of ceramic or glass, and which serves as bases upon which to fabricate the LIMMS device. Various metal conductors (not shown), and which may be of gold (suitably protected as explained below), are deposited on the surfaces of those substrates prior to the application of the patterned dielectric material, or they may be what remains from a patterned removal of an entire metal sheet originally present on the surface of the substrate. The latter case cooperates nicely in instances where some of the conductors are to be co-planar transmission lines formed with the presence of a ground plane. Vias may also be used to connect contact pads within the LIMMS device (i.e., the electrical terminals of the switch) to traces on the other side of a substrate, and to allow traces to pass from one side of a substrate to the other side.
Mercury amalgamates with gold, however, and if enough mercury is present, will dissolve it. It is therefore desirable to protect any gold that will come into contact with the mercury by a protective covering of another metal, such as platinum, that mercury will wet to but that does not interact with mercury. (Owing to the possibility of mercury smears during assembly, a complete over-covering of all the gold may be more desirable than simply covering the exposed pads where the droplet or slug of mercury might be expected to touch the gold during normal operation.) We shall have more to say about the protective covering in due course.
Now note the patterned layers 28 and 30. They are applied over the various conductors and vias of their respective substrates (27, 31), and may be of KQ 150 or KQ 115 thick film dielectric material from Heraeus, or the 4141A/D thick film compositions from DuPont. These are materials that are applied as pastes and then cured under heat at prescribed temperatures for prescribed lengths of time. Depending upon the particular material, they may be applied as an undifferentiated sheet, cured and then patterned (say, by laser or chemical etching) or they may be patterned upon their initial application (via a screening process). In any event, the patterning produces the heater cavities 34/45 and 33/46, the liquid metal channel 50 and their interconnecting passages (36, 37). The figure shows these passages (36, 37) being formed in only one layer (28) of the two layers of dielectric material. This is sufficient, although there could, if desired, also be matching passages in the other layer (30) leading from cavity halves 45 and 46 into the liquid metal channel 50.
The conventional thick film processes used to print patterned layers of the dielectric material allow considerable control over the finished thickness of one or more cured layers of dielectric material (which might be, say, in the range of five to ten thousandths of an inch), and achieving sufficient uniformity of thickness is not a major difficulty. However, there are limits to how thin and how thick an uncured printed layer can be, and it may be necessary to apply (print) multiple layers to achieve a particular overall depth for each of layers 28 and 30. For the KQ material that is to be printed on using a fine mesh (screen) of stainless steel, an individual printed uncured layer is on the order of one to two thousandths of an inch in thickness. The KQ material shrinks in thickness by an amount of about thirty percent during the curing process. It is possible to print several uncured layers, one on top of the other, and then fire the whole works, or, the application sequence could be print-fire-print-fire . . . , or even print-print . . . print-fire-print-print . . . . During the firing for curing the steep side walls and relatively sharp edges it is possible for the uncured printed layers become sloped and rounded, respectively. The resulting trapezoidal cross-sectional shape of the liquid metal channel 50 may be a significant influence in determining a desired thickness for layer 30. In this connection, the view 26 shown in FIG. 5 is a considerable simplification, in that, for simplicity of the drawing, the heater cavities 45 and 46, liquid metal channel 50, and their interconnecting passages 36 and 37 are all depicted as having steep side walls and sharp edges. It makes the basic subject matter of the drawing much easier to appreciate. When using printed KQ, however, the actual situation is much close to what is shown in FIGS. 6-8. Note the sloping side walls of the various patterned layers of dielectric material. Steep sidewalls and sharp edges are not necessarily bad, and can be obtained with other fabrication techniques, although that may also have an effect on the method used to create metalized regions, such as 47-49 that are to ascend such steep side walls.
Once layer 28 has been formed and patterned, metallic regions 42-44 are deposited over their respective vias (19-21 of FIG. 4, and which are not shown). These metallic regions 42-44 correspond to metallic contacts 22-24 of FIG. 4, and serve to improve electrical contact with the liquid metal and to provide a surface that can be wetted by the liquid metal (for latching). In similar fashion, metallic regions 47-49 may be deposited in the channel 50 to provide additional wetting surface for keeping the mercury in place between switching operations. (Regions 47-49 are not expected nor required to be in electrical contact with their corresponding regions 42-44.)
Note the heater resistors 34 (shown in place) and 35 (shown exploded above its intended location). On the tops of opposing edges of their respective heater cavities (32 and 33) are pair of metallic strips (40/41 and 38/39) that cover and connect to heater drive vias (not shown, but correspond to the likes of 17 and 18 in FIG. 4). In this manner the heater resistors 34 and 35 are suspended above the substrate for greater thermal efficiency, faster heating time and reduced electrical power consumption.
If desired, strips of metal may be applied around patterned layers 28 and 30 at the perimeter of the LIMMS device. Such strips are part of an hermetic seal that is formed of solder. Glass frit may also be used as a sealant, in which case the metal strip around the perimeter is not required. The hermetic seal may also involve there being beveled edges along the perimeter that receive the metal or frit. Some examples will be given later in connection with FIGS. 8 and 9. In any event, on the top surface of the patterned layer 38 of dielectric material is applied a correspondingly patterned layer 29 of adhesive, such as CYTOP. The patterning of the adhesive layer 29 matches the various features of the dielectric layers 28 and 30 that are to mate with each other, and for clarity is shown exploded away from patterned dielectric layer 28.
To assemble the LIMMS shown in view 26 of FIG. 5, the top-half (31/30) is turned upside-down. The channel 50 receives its droplets of liquid metal (not shown) and, while in an atmosphere of a suitable gas, such as N2, the upside-down bottom half (27/28) would be registered and affixed against the upside-down top half (31/30). Then the hermetic seal would be formed.
We are always interested in techniques that improve device capability, reduce device fabrication cost, reduce the costs associated with connecting the device to a surrounding circuit, or increase the number of devices in a package without increasing the size of the footprint of the package. Increasing the number of LIMMS devices within a given footprint. has, it will be noted, the potential of improving device capability by both offering greater functionality for the hybrid as a whole (more switches means more things can be done) and better performance arising from shorter signal paths. Performance and cost both benefit from the reduced use of hybrid-to-hybrid interconnections achieved by putting more stuff onto one hybrid. The use on the bottom substrate for a LIMMS device of a patterned layer of dielectric forming cavities, channels and interconnecting passages is an attractive starting point. But even then, complex arrangements of LIMMS devices can spread out with increasing footprints and can also present trace routing problems. What to do?
An attractive solution to the problem of increasing the number of LIMMS devices in an assembly while minimizing the increase in the footprint of the assembly is to stack multiple layers of LIMMS devices on top of one another, and interconnect those layers at an array of solder pads using solder balls. Each layer includes a pair of substrates between which are formed the actual LIMMS devices themselves. The layers use vias to bring the needed conductors to the array of solder pads. All signals for the entire multi-layer assembly can be routed through the bottom LIMMS device layer to pass, through another array of solder pads onto a xe2x80x9cmother substratexe2x80x9d of ceramic or other material that carries the assembly. Alternatively, signals may enter or leave the upper LIMMS device by way of a flexible printed circuit harness.
This plan contemplates creating vias that pass, either directly or by xe2x80x9cdog legsxe2x80x9d on interior surfaces, completely through the bottom LIMMS device layer, and through any other LIMMS device layers, as needed. Such xe2x80x9cthrough the device layerxe2x80x9d (of two substrates) vias are formed of two opposing vias having pads that do not touch but that are bridged by a small ball of liquid metal held in place by a hole in the surrounding dielectric material. It also contemplates traces that run horizontally within the interior of a LIMMS device layer. Using patterned layers of dielectric to form holes for liquid metal balls that join opposing vias, cavities, channels and interconnecting passages for the LIMMS devices of both layers facilitates these needed vias and traces. The use of such patterned layers itself depends upon the use of a suitable dielectric material, which must be strong, adheres well to the substrate, is impervious to contaminants, is capable of being patterned, and if also desired, which can be metalized for soldering. It should also have well controlled and suitable properties as a dielectric. Given a choice, a lower dielectric constant (K) is preferable over a higher one. Suitable thick film dielectric materials that may be deposited as a paste and subsequently cured include the KQ 150 and KQ 115 thick film dielectrics from Heraeus and the 4141A/D thick film compositions from DuPont.